In line with the recent demand for fuel-efficient cars, a conjugated diene-based polymer having adjustment stability represented by wet skid resistance as well as low rolling resistance and excellent abrasion resistance and tensile properties is required as a rubber material for a tire.
In order to reduce the rolling resistance of a tire, there is a method of reducing a hysteresis loss of a vulcanized rubber, and rebound resilience at 50° C. to 80° C., tan δ, or Goodrich heat generation is used as an evaluation index of the vulcanized rubber. That is, it is desirable to use a rubber material having high rebound resilience at the above temperature or low tan δ or Goodrich heat generation.
A natural rubber, a polyisoprene rubber, or a polybutadiene rubber is known as a rubber material having a low hysteresis loss, but these rubbers may have low wet skid resistance. Thus, recently, a conjugated diene-based (co)polymer, such as a styrene-butadiene rubber (hereinafter, referred to as “SBR”) or a butadiene rubber (hereinafter, referred to as “BR”), is prepared by emulsion polymerization or solution polymerization to be used as a rubber for a tire.
In a case in which the BR or SBR is used as the rubber material for a tire, the BR or SBR is typically used by being blended with a filler, such as silica or carbon black, to obtain physical properties required for a tire. However, since an affinity of the Br or SBR with the filler is poor, physical properties, such as abrasion resistance, crack resistance, or processability, may rather be reduced.
Thus, as a method of increasing dispersibility of the SBR and the filler such as silica or carbon black, a method of modifying a polymerization active site of a conjugated diene-based polymer obtained by anionic polymerization using organolithium with a functional group capable of interacting with the filler has been proposed. For example, a method of modifying a polymerization active end of a conjugated diene-based polymer with a tin-based compound or introducing an amino group, or a method of modifying with an alkoxysilane derivative has been proposed.
Also, as a method of increasing dispersibility of the BR and the filler such as silica or carbon black, a method of modifying a living active terminal with a specific coupling agent or modifier has been developed in a living polymer obtained by coordination polymerization using a catalyst composition which includes a lanthanide rare earth element compound.
However, since the BR or SBR modified by the above-described method has a low terminal modification ratio, a physical property improvement effect was insignificant with respect to a tire prepared by using the same.